The present invention relates to an improved ultrasonic transducer, and more particularly relates to improvement in an ultrasonic transducer incorporating polymer piezoelectrics, which is well suited for ultrasonic diagnostics and other nondestructive evaluations.
In recent years, increasing attention has been paid to piezoelectric polymers such as polyvinylidene fluoride (PVDF) and copolymers of vinylidene fluoride with other components, because they have very remarkable properties different from those of conventional piezoelectrics materials such as PZT or B.sub.a T.sub.i O.sub.3. For example, polymer piezoelectrics have low acoustic impedance close to that of water, plastics, or human bodies, and furthermore, they are flexible, and resistive to mechanical shock. These piezoelectric polymers have relatively strong electromechanical coupling factor k.sub.33.sup.t for thickness extensional mode. Thus, the piezoelectric polymer films can be easily shaped into any desired form, and are very suitable for the transducers for ultrasonic diagnostics or non-destructive evaluations.
Various types of ultrasonic transducers have been proposed, which incorporate polymeric piezoelectrics.
In one simple example of such transducers, a polymer piezoelectric film is sandwiched by a pair of thin electrodes, and is bonded to a suitable holder substrate. By applying electric signals to the electrodes, the transducer radiates ultrasonic waves. The transducer is further able to receive external ultrasonic waves as corresponding electric signals. The transducer of this type, however, is inevitably accompanied by undesirable backward leakage of ultrasonic waves. In order to avoid this disadvantage, various constructions have been devised, which naturally results in undesirable rise in production cost.
In order to avoid this leakage trouble, another example of the conventional transducer includes a reflective layer known as a quarter wave reflector, which is made of high acoustic impedance materials, such as copper, other metals or ceramics. The said layer is interposed between the piezoelectric element and the holder substrate. This well blocks leakage of ultrasonic waves via the holder substrate. However, as described later in more detail, the relatively large thickness of above mentioned reflective layer seriously spoils the very advantage of the polymer piezoelectrics, i.e. high flexibility and excellent easiness in processing. In particular, the increased thickness of the reflective layer disables easy application of etching technique and other fine mechanical treatments to the reflective layer, which is needed in production of, for example, phased-array, linear array, or multi-element transducers.